1. Field of the Invention
The present invention relates to a fabrication method of a solid electrolyti capacitor and more particularly, to a fabrication method of a solid electrolytic capacitor using a conducting polymer (for example, polypyrrole, polythiophene, and polyaniline) as a solid electrolyte.
2. Description of the Prior Art
In recent years the electronic components have been becoming miniaturized more and more and their operation speed and operation frequency have been becoming higher and higher. To cope with this tendency, the performance or characteristic of capacitors have been required to be improved in a high-frequency region.
Typically, a chip-type solid electrolytic capacitor has a porous capacitor body or pallet, which is typically made by sintering a powder or a valve metal such as tantalum (Ta) and aluminum (Al). The porous capacitor body serves as an anode. An oxide layer of the valve metal is formed on the expanded surface of th porous capacitor body. The oxide layer serves as a dielectric. A solid electrolyte layer is formed on the oxide layer. The solid electrolyte layer serves as a cathode. An anode lead is electrically connected to the capacitor body serving as the anode. A cathode lead is electrically connected to the solid electrolyte layer serving as the cathode through an electrically-conductive layer formed on the solid electrolte layer.
The porous capacitor body, the oxide layer, the solid electrolyte layer, the electrically-conductive layer, the anode lead, and the cathode lead are encapsulated by a resin package in such a way that outer parts of the anode and cathode leads protrude from the package.
The solid electrolyte layer has a function of electrically interconnecting the cathode lead with the entire surface of the dielectric (i.e., the oxide layer) formed on the capacitor body. Therefore, from this viewpoint, it is desirable that the solid electrolyte layer is a substance having a high electrical conuctivity. On the other hand, the solid electrolyte layer needs to have a healing function for healing an electrical short due to defects in the dielectric.
Accordingly, a metal, which has a high electrical conductivity, but has no dielectric healing function, cannot be used as the solid electrolyte layer. As a result, conventionally, mangnese doxide (MnO.sub.2) has been popularly used as the solid electrolyte layer, because MnO.sub.2 has a property that it is transformed from an electrical conductor into an insulator due to the heat generated by a short-circuit current.
However, MnO.sub.2 has a problem of a comparatively low electrical conductivity of approximately 0.1 S/cm. To solve this problem, some improved capacitors using a 7,7,8,8,-tetracyanoquinodimethine (TCQN) complex with an improved electrical conductivity as the solid electrolyte layer have been developed.
Recently, various capacitors using one of such conducting polymers as a polypyrole, polyaniline, and polythiophene as the solid electrolyte layer have been vigorously developed. This is because these conducting polymers have an electrical conductivity as high as 10 to 100 S/cm.
It is has been known that these conducting polymer layers can be generated on the oxide layer with the use of an "electrolytic polymerization" or "chemically-oxidative polymerization" process.
First and second examples of the conventional fabrication methods of the solid electrolytic capacitor using the chemically-oxidative polymerization process are shown in FIGS. 1 and 2, which is disclosed in the Japanese Non-Examined Patent Publication No. 4-94110 published in 1992.
In the first conventional example, as shown in FIG. 1, an etched aluminum foil whose surface is covered with a dielectric is impregnated with a solution of an oxidizing agent in the step S101. The impregnated solution of the oxidizing agent is then dried in step S102. Subsequently, the etched aluminum foil is exposed to or contacted with a pyrrole in vapor phase to thereby polymerize the pyrrole by chemically-oxidative polymerization due to the action of the impregnated oxidizing agent. As a result, a polypyrrole layer is formed on the dielectric of the aluminum foil in the step S103.
Following this step S103, the foil is cleaned to remove the remaining oxidizing agent and pyrrole in the step S104, and dried in the step S105. Thus, the polypyrrole layer is formed on the dielectric of the capacitor body as the conducting polymer layer.
In the second conventional example, as shown in FIG. 2, an etched aluminum foil whose surface is covered with a dielectric is impregnated with solution of an oxidizing agent in the step S201. This step S201 is the same as the step S101 in FIG. 1.
Subsequently, the etched aluminum foil is immersed into a water solution of pyrrole to thereby polymerize the pyrrole by chemically-oxidative polymerization due to the action of the impregnated oxidizing agent. As a result, a polypyrrole layer is formed on the dielectric of the aluminum foil in the step S202.
Following the step S202 the foil is cleaned to remove the remaining oxidizing agent and pyrrole in the step S203 and then, dried in the step S204. Thus, the polypyrrole layer is formed on the dielectric of the capacitor body as the conducting polymer layer.
In the steps S101 and S201, for example, a methanol solution of ferric (III) dodecylbezenesulfonate is used as the solution of the oxidizing agent. The viscosity of the oxidizing agent is set at 100 centipoise (cp) or less not to decrease the fabrication efficiency.
A third example of the conventional fabrication methods of the solid electrolytic capacitor using both the electrolytic polymerization and chemically-oxidative polymerization processes is shown in FIG. 3, which is disclosed in the Japanese Non-Examined Patent Publication No. 5-62863 published in 1993.
FIG. 3 shows only the steps of the chemically-oxidative polymerization process in the third example.
In the step S301, an etched aluminum foil whose surface is covered with a dielectric is immersed into a dedoped polyaniline solution. In the step S302, the etched aluminum foil is further immersed into a butanol solution of naphthalenesulfonic acid, thereby polymerize the dedoped polyaniline by chemically-oxidative polymerization due to the action of the impregnated oxidizing agent. Thus, an polyaniline layer is formed on the dielectric of the aluminum foil in the step S302.
In the step S303, the aluminum foil is dried to remove the remaining solutions thereon. Thus, the polyaniline layer is formed on the dielectric of the capacitor body as the conducting polymer layer.
In the step S301, the specific viscosity of the dedoped polyaniline solution is limited to be equal to or higher than 0.3 g/100 ml and lower than 0.4 g/100 ml. An electrolytic polymerization process is further performed after the chemically-oxidative polymerization process.
However, with th above-explained first and second examples of the conventional fabrication methods disclosed in the Japanese Non-Examined Patent Publication No. 4-94110, the viscosity of the solution of the oxidizing agent is set as low. Therefore, there is a problem that the necessary repetition number of the polyemrization processes becomes large to obtain a desired thickness of the conducting polymer layer. This problem is caused by the fact that the low viscosity means the low concentration of the oxidizing agent in the solution and consequently, the amount of the solid electrolyte (i.e., conducting polymer) generated in each polymerization process is small.
To decrease the necessary repetition number of the polymerization processes, the viscosity (i.e., the concentration) of the solution of the oxidizing agent needs to be raised. In this case, however, there arises another problem that the coverage ratio becomes low.
The same problem as above is applied to the third example of the conventional fabrication methods disclosed in the Japanese Non-Examined Patent Publication No. 5-62863.